The present invention relates to a thermal transfer sheet. More particularly, an object of the present invention is to provide a thermal transfer sheet having a cyan color capable of forming a record image excellent in the transferability, color density, sharpness and various types of fastness, particularly dye migration resistance and light fastness.
Various thermal transfer processes are known in the art. Among them, a sublimation transfer process has been practiced which comprises supporting a sublimable dye as a recording agent on a substrate sheet, such as paper, to form a thermal transfer sheet, putting the thermal transfer sheet on a transfer material dyeable with a sublimable dye, for example, a polyester woven fabric, and applying a thermal energy in a pattern form from the back surface of the thermal transfer sheet to transfer the sublimable dye to the transfer material.
In recent years, a proposal has been made on a process wherein various full color images are formed on paper or a plastic film through the use of the above-described sublimation type or thermal transfer system. In this case, a thermal head of a printer is used as heating means, and a number of color dots of three or four colors are transferred to the transfer material, thereby reproducing a full color image of an original by means of the multicolor dots.
Since the color material used is a dye, the image thus formed is very clear and highly transparent, so that the resultant image is excellent in the reproducibility and gradation and the quality of the image is the same as that of an image formed by the conventional offset printing and gravure printing. In this method, it is possible to form an image having a high quality comparable to a full color photographic image.
The most important problem in the thermal transfer process, however, resides in the color density, dye migration resistance and light fastness of the formed image.
Specifically, in the case of high-speed recording, the thermal energy should be applied in a very short period of time on the order of seconds or less, and since the sublimable dye and transfer material are not sufficiently heated in such a short time, an image having a sufficient density can not be formed.
For this reason, in order to cope with the above-described high-speed recording, a sublimable dye having an excellent sublimableness has been developed. Since, however, the molecular weight of a dye having an excellent sublimableness is generally small, the dye migrates in the transfer material or bleeds on the surface thereof with the elapse of time. This gives rise to problems with respect to dye migration resistance and light fastness derived from a small molecular weight, so that the formed image becomes disturbed or unclear or contaminates neighboring articles.
When a sublimable dye having a relatively high molecular weight is used for the purpose of avoiding the above-described problems, in the high-speed recording process, the sublimation rate is so poor that a satisfactory image can not be formed as with the above-described method.